Biochemistry of glomerular basement membrane of the normal and diabetic human

Plasma protein advanced glycation end products, carboxymethyl cysteine, and carboxyethyl cysteine, are elevated and related to nephropathy in patients with diabetes

In Diabetes Mellitus (DM), glucose and the aldehydes glyoxal and methylglyoxal modify free amino groups of lysine and arginine of proteins forming advanced glycation end products (AGEs). Elevated levels of these AGEs are implicated in diabetic complications including nephropathy. Our objective was to measure carboxymethyl cysteine (CMC) and carboxyethyl cysteine (CEC), AGEs formed by modification of free cysteine sulfhydryl groups of proteins by these aldehydes, in plasma proteins of patients with diabetes, and investigate their association with the albumin creatinine ratio (ACR, urine albumin (mg)/creatinine (mmol)), an indicator of nephropathy. Blood was collected from forty-two patients with type 1 and 2 diabetes (18-36 years) and eighteen individuals without diabetes (17-35 years). A liquid chromatography-mass spectrophotometric method was developed to measure plasma protein CMC and CEC levels. Values for ACR and hemoglobin A1C (HbA1C) were obtained. Mean plasma CMC (microg/l) and CEC (microg/l) were significantly higher in DM (55.73 +/- 29.43, 521.47 +/- 239.13, respectively) compared to controls (24.25 +/- 10.26, 262.85 +/- 132.02, respectively). In patients with diabetes CMC and CEC were positively correlated with ACR, as was HbA1C. Further, CMC or CEC in combination with HbA1C were better predictors of nephropathy than any one of these variables alone. These results suggest that glucose, glyoxal, and methylglyoxal may all be involved in the etiology of diabetic nephropathy.

Increases in renal epsilon-(gamma-glutamyl)-lysine crosslinks result from compartment-specific changes in tissue transglutaminase in early experimental diabetic nephropathy: pathologic implications

Diabetic nephropathy (DN) is characterized by an early, progressive expansion and sclerosis of the glomerular mesangium leading to glomerulosclerosis. This is associated with parallel fibrosis of the renal interstitium. In experimental renal scarring, the protein cross-linking enzyme, tissue transglutaminase (tTg), is up-regulated and externalized causing an increase in its crosslink product, epsilon-(gamma-glutamyl)-lysine, in the extracellular space. This potentially contributes to the extracellular matrix (ECM) accumulation central to tissue fibrosis by increasing deposition and inhibiting breakdown. We investigated if a similar mechanism may contribute to the ECM expansion characteristic of DN using the rat streptozotocin model over 120 days. Whole kidney epsilon-(gamma-glutamyl)-lysine (HPLC analysis) was significantly increased from Day 90 (+337%) and peaked at Day 120 (+650%) (p < 0.05). Immunofluorescence showed this increase to be predominantly extracellular in the peritubular interstitial space, but also in individual glomeruli. Total kidney transglutaminase (Tg) was not elevated. However, using a Tg in situ activity assay, increased Tg was detected in both the extracellular interstitial space and glomeruli by Day 60, with a maximal 53% increase at Day 120 (p < 0.05). Using a specific anti-tTg antibody, immunohistochemistry showed a similar increase in extracellular enzyme in the interstitium and glomeruli. To biochemically characterize glomerular changes, glomeruli were isolated by selective sieving. In line with whole kidney measurement, there was an increase in glomerular epsilon-(gamma-glutamyl) lysine (+361%); however, in the glomeruli this was associated with increases in Tg activity (+228%) and tTg antigen by Western blotting (+215%). Importantly, the ratio of glomerular epsilon-(gamma-glutamyl) lysine to hydroxyproline increased by 2.2-fold. In DN, changes in the kidney result in increased translocation of tTg to the extracellular environment where high Ca(2+) and low GTP levels allow its activation. In the tubulointerstitium this is independent of increased tTg production, but dependent in the glomerulus. This leads to excessive ECM cross-linking, contributing to the renal fibrosis characteristic of progressive DN.

Relationship between fascicle size and perineurial collagen IV content in diabetic and control human peripheral nerve

AIM

The relationship between perineurial collagen IV content and fascicle size was determined in diabetic and control human peripheral nerve.

METHODS AND RESULTS

Age-matched diabetic and control sural nerve samples were immunostained using antibodies to collagen IV. The number of cell layers and the perimeter of the fascicle were measured and the collagen IV content of the perineurium determined. Using this method, a comparison could be made of collagen IV content in the perineuria of fascicles of different size. A positive linear relationship was found between fascicle size and the amount of collagen IV per unit of perineurium. The number of perineurial cell layers and the collagen IV content of the diabetic nerve did not differ from control values.

CONCLUSIONS

The linear relationship between fascicle size and perineurial collagen IV content per unit of perineurium underlines the importance of taking fascicle size into account when determining changes in basement membrane components associated with neuropathy. The results indicate that increased deposition of collagen IV is not involved in the early changes in the perineurial cell basement membrane and is thus not the primary factor involved in altered nerve function associated with diabetic neuropathy.

Renal disease and hypertension in non-insulin-dependent diabetes mellitus

Recent epidemiologic data demonstrate a dramatic increase in the incidence of end-stage renal disease (ESRD) in patients with non-insulin-dependent diabetes mellitus (NIDDM), thus dispelling the mistaken belief that renal prognosis is benign in NIDDM. Currently, the leading cause of ESRD in the United States, Japan, and in most industrialized Europe is NIDDM, accounting for nearly 90% of all cases of diabetes. In addition to profound economic costs, patients with NIDDM and diabetic nephropathy have a dramatically increased morbidity and premature mortality. NIDDM-related nephropathy varies widely among racial and ethnic groups, genders and lifestyles; and gender may interact with race to affect the disease progression. While the course of insulin-dependent diabetes mellitus (IDDM) progresses through well-defined stages, the natural history of NIDDM is less well characterized. NIDDM patients with coronary heart disease have a higher urinary albumin excretion rate at the time of diagnosis and follow-up. This greater risk may also be associated with hypertension and hyperlipidemia, and genes involved in blood pressure are obvious candidate genes for diabetic nephropathy. Hyperglycemia appears to be an important factor in the development of proteinuria in NIDDM, but its role and the influence of diet are not yet clear. Tobacco smoking can also be deleterious to the diabetic patient, and is also associated with disease progression. Maintaining euglycemia, stopping smoking and controlling blood pressure may prevent or slow the progression of NIDDM-related nephropathy and reduce extrarenal injury. Treatment recommendations include early screening for hyperlipidemia, appropriate exercise and a healthy diet. Cornerstones of management should also include: (1) educating the medical community and more widely disseminating data supporting the value of early treatment of microalbuminuria; (2) developing a comprehensive, multidisciplinary team approach that involves physicians, nurses, diabetes educators and behavioral therapists; and (3) intensifying research in this field.

Alterations of glomerular basement membrane charge and structure in diabetic nephropathy

We examined glomerular basement membrane anionic site distribution identified by cationic gold in seven patients with insulin-dependent and four patients with non-insulin-dependent diabetes mellitus, presenting a spectrum of clinical and glomerular changes. Anionic sites were investigated by pretreatment of tissue with glycosaminoglycan-degrading enzymes prior to cationic gold staining. The distribution of chondroitin sulphate proteoglycans--a previously unrecognized glomerular basement membrane component--and type IV collagen was examined by immunoelectron microscopy to identify structural changes in the basement membrane. Findings were compared with those of non-diabetic patients showing minor proteinuria and morphologically normal glomerular basement membranes. Two patients, originally diagnosed as having diabetic nephropathy were also examined at 19 weeks and 5 years after renal transplantation. Characteristic redistribution of type IV collagen and chondroitin sulphate proteoglycans was noted in thickened glomerular basement membrane segments (> 400 nm) of diabetic patients and those with renal transplants. Extension of anionic sites deep into the glomerular basement membrane at pH 2.5, together with loss of interna sites at pH 5.8 is unique to diabetic nephropathy. Reduced charge density was apparent in some patients due to thickening of the glomerular basement membrane, although the number of anionic sites per unit length of membrane was actually increased. Thus, charge aberration in diabetic nephropathy is due to displacement rather than loss of anionic sites. Removal of more than 90% of these sites by heparitinase, confirms their association with heparan sulphate proteoglycans. Similar derangement of anionic sites in all patients with diabetic nephropathy irrespective of the degree of proteinuria, suggests that a heparan sulphate proteoglycan-related charge barrier plays a minor role in controlling permeability of the diabetic glomerular basement membrane.

Microalbuminuria: prognostic and therapeutic implications in diabetic and hypertensive pregnancy

Microalbuminuria is defined as urinary excretion of albumin that is persistently above normal, although below the sensitivity of conventional semiquantative test strips. Several studies have reported that Type 1 diabetic patients with microalbuminuria are apparently more likely to develop diabetic nephropathy eventually progressing to renal failure. Microalbuminuria is also a strong predictor of mortality in Type 2 diabetes, and is correlated with increased blood pressure in patients with benign essential hypertension. Radioimmunoassay revealed a significantly higher urinary albumin excretion rate in normal pregnant women in the third trimester of pregnancy, compared to the second and first, and compared to non-pregnant women. Microalbuminuria was found in 30% of women who had a record of gestational diabetes mellitus. Published results are controversial regarding the assumption that microalbuminuria is an early predictor of pregnancy-induced hypertensive complications.

Serum sialic acid concentration is elevated in IDDM especially in early diabetic nephropathy

OBJECTIVES

Elevated serum sialic acid concentration is a strong predictor of cardiovascular mortality in non-diabetic subjects. Because patients with insulin-dependent diabetes mellitus (IDDM) and albuminuria have a highly increased cardiovascular morbidity and mortality, we hypothesized that IDDM patients with albuminuria would have an increased concentration of serum sialic acid.

DESIGN

Cross-sectional study.

SETTING

Outpatient clinic at Steno Diabetes Centre, Gentofte, Denmark.

SUBJECTS

Twenty-six non-diabetic controls and 74 IDDM patients with normoalbuminuria (urinary albumin excretion [UAE] < 30 mg 24 h-1; n = 37), incipient nephropathy (UAE 30-300 mg 24 h-1; n = 20) and clinical nephropathy (UAE > 300 mg 24 h-1; n = 17), matched for sex, age and body mass index (BMI).

MAIN OUTCOME MEASURES

Serum sialic acid concentration, concurrent fasting blood glucose, glycated haemoglobin (HbA1c), serum creatinine, plasma fibrinogen and erythrocyte sedimentation rate.

RESULTS

Normoalbuminuric patients had a higher serum sialic acid concentration (mmol L-1) than non-diabetic controls (1.83 +/- 0.24 vs. 1.67 +/- 0.26; P < 0.02). Serum sialic acid concentration was further increased in patients with incipient nephropathy (2.02 +/- 0.37; P < 0.03) and in patients with clinical nephropathy (2.13 +/- 0.33; P < 0.002) compared with normoalbuminuric IDDM patients. Serum sialic acid correlated strongly with plasma fibrinogen (r = 0.78; P < 0.0001) and erythrocyte sedimentation rate (r = 0.62; P < 0.0001). In a multiple regression analysis including UAE, retinopathy status, fasting blood glucose, HbA1c, mean blood pressure, serum creatinine, age, BMI, duration and smoking. UAE and fasting blood glucose were the independent variables which correlated significantly with serum sialic acid concentration (P < 0.0001 and P < 0.05, respectively).

CONCLUSION

Serum sialic acid is elevated in IDDM especially in albuminuric patients. Whether elevated serum sialic acid is predictive for early diabetic nephropathy and cardiovascular disease in IDDM has to be shown in the future.

High-resolution ultrastructural study of the rat glomerular basement membrane in long-term experimental diabetes

The ultrastructure of the glomerular basement membrane of the long-term diabetic and age-matched control rats was studied with the application of advanced high-resolution microscopy. By using the freeze substitution method for the preparation of the renal tissue, it was possible to observe that the glomerular basement membrane in control and diabetic animals is composed on only a single lamina densa without the presence of a lamina lucida interna or externa. High-resolution electron microscopy of the diabetic glomerular basement membrane showed significant alterations in its morphology and ultrastructure. First, the basement membrane in diabetic condition appeared to be split into two halves, endothelial and epithelial. In the epithelial half of the membrane, the network of distinct strands referred to as cords, which were clearly present in the glomerular basement membrane of age-matched control animals, became less distinct and showed a diffused appearance being evenly replaced by thin filaments. The openings of the network were filled with a granular material. In the endothelial half of the membrane, on the other hand, the cord network was variably lost in diabetic condition and, within the resulting vacant spaces, bundles of fibrils 12 nm in width, identified as basotubules, were deposited. Immunolabeling for type IV collagen was found to be enriched in the endothelial half of the basement membrane being associated with the bundles of basotubules. The ultrastructural changes reported by high-resolution microscopy could be related to the molecular alterations of the basement membrane components and to the loss in permselectivity occurring during diabetes.

Post-exercise albuminuria does not predict microalbuminuria in type 1 diabetic patients

To investigate whether post-exercise urinary albumin excretion in Type 1 diabetic children and adolescents may prospectively predict the development of microalbuminuria, we have assessed post-exercise urinary albumin excretion before and after 6.2 +/- 1.7 years of follow-up in 66 diabetic children and adolescents. Post-exercise urinary albumin excretion rose significantly above the pre-exercise values in diabetic patients by 2.7 (-3.8 to 84.2) micrograms min-1 (p < 0.001) and in a group of 9 healthy individuals by 3.9 (-0.7 to 13.7) micrograms min-1 (p < 0.02) without significant differences between groups. Post-exercise albuminuria was greater in postpubertal than prepubertal 9.8 vs 4.3 micrograms min-1 (p < 0.03) and pubertal 9.8 vs 6.0 micrograms min-1 (p < 0.02) patients; post-exercise changes in urinary albumin excretion were also positively related to glycated haemoglobin (r = 0.293; p < 0.05). Eight out of 66 patients developed microalbuminuria at follow-up. Urinary albumin excretion at follow-up was comparable between patients with normal and abnormal post-exercise urinary albumin excretion; moreover post-exercise urinary albumin excretion was within the normal range in 5 out of 8 patients with microalbuminuria at follow-up. In conclusion post-exercise albuminuria does not seem to be a useful predictor of the onset of microalbuminuria in Type 1 diabetic children and adolescents.

Renal tubular basement membrane and collagen type IV in diabetes mellitus

The pathogenesis of the multiple structural lesions in diabetic nephropathy remains debated, and likely is multifactorial. The uniform thickening of the renal basement membranes lining the glomerular and tubular elements appears to be a consequence of the metabolic perturbations which are directly related to hyperglycemia. While most investigations have focused on the increased accumulation of extracellular matrix in the glomerular basement membrane and the mesangium, and their relation to derangements in glomerular function, little is known regarding the pathogenesis and the significance of the tubulointerstitial changes and the thickened tubular basement membrane (TBM). It is possible that these latter changes are causally related to the cellular hypertrophy of the renal tubular epithelium that lines the TBM. It has been postulated that in the earlier stages of the disease, hyperglycemia induces renal tubular hypertrophy and stimulates the synthesis of the various matrix components which are normal constituents of the TBM. Later, the structural composition of the TBM is susceptible to further modifications by non-enzymatic glycation, and this aberrant process may impart a relative resistance to matrix degradation leading to a slow turnover. In vitro investigations on murine proximal tubule cells in culture have provided evidence that elevated ambient glucose is a sufficient stimulus for cellular hypertrophy and increased biosynthesis of collagen type IV, the predominant constituent of TBM. High glucose levels increase steady-state collagen IV mRNA, partly due to transcriptional activation of cis-acting elements of the gene which are controlled by putative glucose-responsive trans-acting proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Glomerular charge selectivity and the influence of improved blood glucose control in type 1 (insulin-dependent) diabetic patients with microalbuminuria

We first compared glomerular charge selectivity index in two matched groups of Type 1 (insulin-dependent) diabetic patients with micro- and normoalbuminuria respectively, and secondly, investigated prospectively in a randomized clinical trial, the influence of improved metabolic control on selectivity index in diabetic patients with microalbuminuria. In Study 1, 27 patients with microalbuminuria (albumin excretion > or = 15 micrograms/min in at least two out of three overnight urine samples) were matched (age, diabetes duration, mean 1-year HbA1c, gender) with normoalbuminuria patients (n = 24), and in Study 2, 23 microalbuminuric patients were randomly allocated to either intensive (continuous subcutaneous insulin infusion) or conventional treatment. Glomerular charge selectivity index was measured as IgG/IgG4 selectivity index, i.e. total IgG/IgG4 clearance ratio in timed overnight urine samples. The microalbuminuric patients had a significantly reduced selectivity index compared to the normoalbuminuric patients: 1.20 (0.92-1.40) vs 1.68 (1.22-2.21), median and 95% confidence interval (p < 0.01). In Study 2, the HbA1c improved in the intensive-treatment group compared to the conventional-treatment group: at 2, 6 and 12 months the difference in mean percentage HbA1c between the groups was 1.1, 1.2 and 1.4, respectively (p < 0.01). A sharp 50% increment in IgG/IgG4 selectivity index was seen in the intensive-treatment group during the first 6 months (p < 0.05 compared to the conventional group). We conclude that adolescents and young adults in an early stage of diabetic nephropathy have reduced glomerular charge selectivity, which may be improved by reducing the mean blood glucose level.

The increase in albuminuria induced by exercise is not associated with preferential excretion of glycated albumin in type 1 (insulin-dependent) diabetic patients

The increase in urinary albumin excretion induced by acute exercise in Type 1 (insulin-dependent) diabetic patients is associated with the urinary excretion of cationic proteins. To test whether the renal excretion of glycated albumin (more anionic than non-glycated albumin) is affected by exercise, we submitted seven normoalbuminuric (albumin excretion rate less than 30 mg/24 h) Type 1 diabetic patients and six well-matched healthy subjects to an exercise test (600 kpm/min for 20 min) on a bicycle ergometer, preceded and followed by a 1-h resting period. The selectivity index (renal clearance of non-glycated/glycated albumin) was not significantly different among the pre-exercise, exercise and post-exercise periods, either in the normal subjects (1.01 +/- 0.03 vs 1.08 +/- 0.06 vs 1.08 +/- 0.05) or in the diabetic patients (1.25 +/- 0.09 vs 1.20 +/- 0.07 vs 1.20 +/- 0.06), whereas it was significantly higher (p less than 0.05) in diabetic patients compared to healthy subjects during pre-exercise. These results are not consistent with the hypothesis that acute exercise may induce a preferential excretion of glycated albumin.

Potentiation of diabetic glomerulopathy in uninephrectomized rats subchronically exposed to cadmium

The renal effects of diabetes mellitus and cadmium (Cd), separately or in combination, were investigated in unilaterally nephrectomized female Sprague-Dawley rats. Diabetes was induced by injection of streptozotocin and Cd was administered in drinking water at a concentration of 100 p.p.m. for 2.5 months. Cd did not affect the reduction in glomerular filtration rate or the rise in beta 2-microglobulinuria caused by diabetes. By contrast, the effect of diabetes on the urinary excretion of albumin, transferrin or IgG was greatly enhanced by concomitant exposure to Cd. This interaction occurred at Cd levels in the renal cortex which are very similar to those found in the general population of industrialized countries. These observations, in agreement with the results of a recent epidemiological study, suggest that Cd polluting the environment might potentiate the development of diabetic nephropathy.

Urinary excretion of glycated albumin in insulin-dependent diabetic patients with normal urinary albumin excretion

Glycation involves both circulating proteins, such as albumin, and structural proteins, such as the components of the glomerular basement membrane. A preferential excretion of glycated albumin (more anionic at physiological pH compared with unmodified plasma albumin) has been reported by some authors, but not by others. We therefore investigated the selectivity index (renal clearance of non-glycated albumin/clearance of glycated albumin) in 25 insulin-dependent diabetic patients with normal urinary albumin excretion and in 19 well-matched control subjects. The selectivity index was significantly higher in diabetic patients than in control subjects: 1.38 +/- 0.05 SEM vs 0.98 +/- 0.02, p less than 0.0001. This result is not consistent with a preferential urinary excretion of glycated albumin, at least in normoalbuminuric uncomplicated insulin-dependent diabetic patients.

Proteinuria: changes and mechanisms in toxic nephropathies

During the last few decades, considerable progress has been made in the understanding of the pathophysiological mechanisms of proteinuria. A great variety of hemodynamic or biochemical mechanisms acting at different sites of the nephron have been shown to alter the renal handling and the urinary excretion of proteins. The finding which perhaps has had most practical implications is that the pattern of protein excretion quantitatively and qualitatively varies with the site and severity of renal damage. This has led to the development of a large array of methods for the identification and quantitation of specific urinary proteins. These methods have been most extensively used by toxicologists in experimental, epidemiological, or clinical studies on potentially nephrotoxic chemicals (e.g., drugs, heavy metals, solvents, etc.). The present review summarizes the current state of knowledge on the mechanisms of proteinuria and the use of urinary proteins as indicators of nephrotoxicity.

Immunogold studies of monomeric elements from the globular domain (NC1) of type IV collagen in renal basement membranes during experimental diabetes in the rat

The protein A-gold immunocytochemical technique was applied to reveal the monomeric elements M1, M2* and M3 from the non-collagenous globular domain (NC1) of type IV collagen over various renal basement membranes from control and long-term streptozotocin-induced diabetic rats. This study includes the basement membranes of the proximal tubule, the Bowman's capsule and the glomerulus as well as the extracellular matrix of the mesangium. The labellings obtained were confined to basement membrane material. The quantitative analysis demonstrated changes in labelling intensities and distribution between tissues from normal and diabetic animals. Increased labelling intensities were observed for M1 and M2* monomers in all the basement membranes studied except for the mesangial matrix which remained unchanged. In addition, the labelling for M1 monomers, present on the endothelial side of the glomerular basement membrane of control animals, was found to be distributed throughout the entire thickness of the basement membrane of diabetic animals. In contrast, neither the intensity of the labelling, nor the distribution of M3 monomers were altered in diabetic animals. Since M1 monomers are markers of the alpha 1(IV) and alpha 2(IV) chains of type IV collagen while M2* and M3 mark alpha 3(IV) and alpha 4(IV) chains respectively, the present results demonstrate changes in the nature of the collagenous elements of basement membranes during diabetes. Furthermore, the results indicate that the alpha 3(IV) and the alpha 4(IV) chains are not necessarily present in the same molecule. The modifications of the collagenous elements of the basement membranes during diabetes must alter the structural characteristics of these matrices which in turn might influence their functional properties.

Urinary proteins and red blood cell membrane negative charges in diabetes mellitus

The nature and origin of proteinuria in diabetes mellitus have been investigated by measuring the urinary excretion of seven specific proteins of low (beta 2-microglobin, retinol-binding protein) or high molecular weight (albumin, transferrin, hemopexin and IgG). Using the Alcian Blue binding test, we also measured negative charges on red blood cell (RBC) membrane which according to recent studies might mirror the glomerular polyanion charge. A group of 190 diabetics was examined, including 90 patients with type I diabetes, 23 type II diabetics treated with diet and/or hypoglycaemic agents and 77 longstanding type II diabetics requiring insulin therapy. With the exception of beta 2-microglobulin all proteins measured were excreted in the urine of diabetics in significantly higher amounts than in controls. The assay of transferrin proved the most sensitive (58% positive) followed by albumin (49%), IgG (34%), hemopexin (28%) and retinol-binding protein (26%). Practically the same ranking was obtained when only type I diabetics were considered. RBC membrane negative charges were diminished in diabetics and negatively correlated with the urinary excretion of albumin (r = -0.61, n = 190). RBC charges were also negatively correlated with other urinary proteins of high molecular mass (r between - 0.5 and - 0.2) but presented no relation with urinary beta 2-microglobulin or retinol-binding protein. The loss of RBC charges in diabetics most likely reflects the concomitant depletion of the glomerular polyanion responsible for the increased glomerular leakage of high molecular mass plasma proteins. The preferential increase in transferrin excretion together with the progressive rise in the urinary excretion of IgG lead us to postulate that the loss of glomerular polyanion in diabetes is accompanied, from the early stage, by a progressive decrease in the size-selectivity of the glomerular filter. The urinary excretion of retinol-binding protein was weakly correlated with albuminuria (r = 0.26, n = 186). Eight % of diabetics showed an elevation of urinary retinol-binding protein without evidence of microalbuminuria, which clearly demonstrates that a proximal tubular impairment can occur independently of the glomerular alterations in the course of diabetic nephropathy.

Kidney sialidase and sialyltransferase activities in spontaneously and experimentally diabetic rats. Influence of insulin and sorbinil treatments

Kidney cortex sialic acid level, sialidase and sialyltransferase activities have been measured in spontaneously diabetic BB rats and in streptozotocin-diabetic rats (STZ). In untreated diabetic BB rats, at the onset of the disease, sialidase specific activity was found to be increased by 21% when compared with diabetes-resistant BB controls (P less than 0.05) whereas sialyltransferase activity was not significantly modified and bound sialic acid concentration was diminished (P less than 0.05). In diabetic BB rats submitted to a minimal insulin therapy, during 3 months of disease, sialidase activity and sialic acid concentration were similar to those of Wistar age-matched controls. In STZ-diabetic Wistar rats, sialidase specific activity was increased by 76% after 5 months of disease when compared to age-matched Wistar controls (P less than 0.01); in contrast, specific sialyltransferase activity was decreased by 21% (P less than 0.05); these enzymatic alterations were associated with a decrease in bound sialic acid concentration (P less than 0.01); 1 month's insulin therapy, started 4 months after onset of the disease, normalized sialidase activity but had no effect on sialyltransferase activity and sialic acid concentration; treatment with sorbinil prevented cataract development but had no effect on sialidase activity whereas it emphasized the decrease in sialyltransferase activity and sialic acid concentration. The disturbances in the enzyme activities concerned with sialoglycoconjugate metabolism observed in experimental and spontaneous diabetes may be responsible for the decreased bound sialic acid content observed in the rat kidney cortex.

Electrical anionic charges on red blood cells are reduced in insulin-dependent diabetic patients

The glomerular polyanion represents a charge-selective filter, restricting the passage of negatively charged plasma proteins, such as albumin. A specific loss of charges on the glomerular capillary wall (GCW) associated with an increased clearance of albumin has been reported in diabetes mellitus as well as many other nephropathies. The authors performed a chemical test based on the binding of the cationic dye Alcian Blue 8GX (AB) in 19 insulin-dependent diabetics, aged 34 +/- 4 years; all were Albustix negative, with normal renal function, normotensive, and in good metabolic control. The authors also studied 19 well-matched healthy controls. Each subject repeated the test on two separate occasions. The mean AB binding value was significantly lower (t-test: p less than 0.01) in diabetics than in controls: 87.5 +/- 2.3 vs. 100.5 +/- 1.6 ng/10(6) red cells. AB binding was not significantly correlated with age, duration of diabetes or metabolic control. A negative correlation was found between the negative charge on red cells and albumin excretion rate in microalbuminuric patients (n = 9). These results suggest a possible role for electrical charges in early stages of diabetic nephropathy.

New parameters to monitor the progression of diabetic nephropathy

The possible differential elimination of the anionic IgG4 and of the other cationic IgG molecules whose pH differs but whose other characteristics are similar, has been hypothesized as a possibly useful parameter in monitoring preclinical diabetic nephropathy. An enzyme-linked immunosorbent assay method has been developed, based on a sandwich technique with subclass-specific antiimmunoglobulin monoclonal antibodies, which detects about 2 ng/mL IgG4. A sensitive radioimmunoassay method has been used to detect IgG. Normoalbuminuric, microalbuminuric, and macroalbuminuric patients, together with normal control subjects, were included in the cross-sectional study. Whereas IgG levels were elevated, as expected, in macroalbuminuric patients, it was interesting to note that IgG4, but not total IgG, levels were elevated in microalbuminuric patients. The IgG4/IgG ratio was increased almost to the same extent in microalbuminuric and macroalbuminuric patients. These findings are strongly in favor of the selective elimination of the acid medium-sized protein, IgG4, in incipient diabetic nephropathy. The measurement of immunoglobulin subclasses in the urine appears to be a promising parameter to characterize and subgroup diabetic patients with preclinical diabetic nephropathy.

Effect of the exercise test on albuminuria, blood pressure and blood glucose in type I (insulin-dependent) diabetic patients

Twenty-nine male type I diabetic patients (age range 16-46 years) and thirteen healthy men (age range 18-43 years) were exercised on a cycling ergometer at 75 W and 100 W after having achieved a steady state of water diuresis. Diabetic patients were subdivided into Group A (n = 19, resting urinary albumin excretion rate - UAER less than or equal to 16 micrograms/min) and Group B (n = 10, 16 less than resting UAER less than 126 micrograms/min). The groups were comparable in weight, serum creatinine, duration of diabetes and glycosylated hemoglobin. Group B showed the highest elevation of UAER at the work load of 100 W, with no correlation between increase in UAER and increase in systolic blood pressure (SBP) at both work loads. The only correlation between these parameters was found in Group A at the work load of 100 W (p less than 0.05). No correlation was found between exercise UAER and actual blood glucose in either group. The difference in UAER between healthy subjects and Group B patients (p less than 0.001) remained on the same level during exercise as at rest, but the difference between Group A and Group B (p less than 0.001) decreased with increasing work load (p less than 0.05). The highest exercise-induced systolic and diastolic blood pressure (DBP) was found in Group B, although there was no difference between the diabetic groups in pre-exercise blood pressure and in mean SBP and DBP from previous outpatient check-ups. Blood glucose did not change significantly during exercise in either diabetic group. Working capacity of diabetic patients was lower than that of healthy subjects. The test revealed some diabetic patients with strong elevation of UAER and with abnormally raised systolic and diastolic BP during exercise. The value of the findings reported is to be clarified in a further longitudinal study.

Glomerular size and charge selectivity in insulin-dependent diabetes mellitus

The pathogenesis of clinical nephropathy in Type 1 (insulin-dependent) diabetes was investigated by measuring renal fractional clearances of albumin, total IgG, IgG4 and beta 2-microglobulin, four plasma proteins which differ in size and charge. Seventy patients and eleven control subjects were studied. In diabetic patients with normal urinary albumin excretion (less than 30 mg/24 hr), fractional IgG clearance was two to three times higher than in control subjects, whereas fractional clearance of the anionic plasma proteins IgG4 and albumin was similar to that of control subjects. These alterations indicate an increase in anionic pore charge within the glomerular basement membrane concomitant with an increase in either pore size or impairment of tubular reabsorption. Diabetic patients, whose urinary albumin excretion has started to rise (30 to 100 mg/24 hr), had unchanged fractional IgG compared to patients with normal albumin excretion, while fractional IgG4 and albumin clearances were increased three- to fourfold; indicating unchanged glomerular pore size, but a decrease in anionic pore charge. In patients demonstrating urinary albumin excretion of greater than 100 mg/24 hr fractional IgG clearance increased to the same extent as fractional albumin clearance, indicating an increase in large pore area. Fractional beta 2-microglobulin clearances were similar to that of control subjects in the different patient groups indicating unchanged tubular reabsorption of proteins. Thus, the increase in large pore area seen in patients with clinical nephropathy is preceded by loss of anionic charge in the glomerular basement membrane. It is likely that this loss of anionic charge is due to loss of heparan sulphate-proteoglycan.

Diabetes mellitus associated changes in glomerular glycocompounds: a fluorescence microscopical study

Renal biopsy specimens showing histological alterations typical of advanced diabetic glomerulopathy were studied for changes in glomerular glycoconjugates, using fluorochrome-coupled lectins as probes. All samples studied showed a marked reduction in the binding of Triticum vulgaris (WGA) lectin in the glomerular basement membranes. On the other hand, 'new' glomerular binding sites for the lectins of Dolichos biflorus (DBA), Helix pomatia (HPA) and Arachis hypogaea (PNA), recognizing galactosyl moieties of glycoconjugates and giving no reaction in normal glomeruli, were seen in all samples studied. In addition, Wistaria floribunda lectin (WFA), recognizing galactosyl and, N-acetylgalactosaminyl configurations in glycoconjugates, gave a typical linear binding along the glomerular basement membranes, differing markedly from its reaction with normal kidney. Ulex europaeus (UEA I) showed reduced binding in the glomeruli of diabetic nephropathy. The results show that changes in glomerular glycoconjugates may appear in diabetic nephropathy, suggesting a disturbance in the turnover of the non-reducing terminal saccharide residues. In addition, the results show that lectins are useful probes for studying these changes further.

Evidence of changes in renal charge selectivity in patients with type 1 (insulin-dependent) diabetes mellitus

Altered filtration of macromolecules due to decreased electrical charge of the glomerular basement membrane might be the initial step in the development of albuminuria in patients with Type 1 (insulin-dependent) diabetes mellitus. We therefore investigated the selectivity index, i.e. renal clearance of non-glycated plasma albumin/clearance of glycated plasma albumin in 38 patients with Type 1 diabetes mellitus. The two albumin molecules differed slightly in charge, non-enzymatic glycated albumin being more anionic at physiological pH compared with unmodified plasma albumin. Glycated albumin in plasma and urine was determined by a specific, sensitive and highly reproducible chromatographic procedure. In diabetic patients with normal urinary albumin excretion, the selectivity index was increased three-fold compared with that of non-diabetic subjects (2 p less than 0.01). A significant correlation (r = 0.53, 2 p less than 0.01) between haemoglobin A1c and selectivity index was demonstrated in these patients, indicating a change in charge-dependent renal filtration could possibly be attributed to non-enzymatic glycation of components in the glomerular basement membrane and tubuli. Diabetic patients with increased albumin excretion rate had a significantly lower selectivity index compared with patients with normal albumin excretion (2 p less than 0.01). A significant negative correlation (r = 0.85, 2 p less than 0.001, exponential curve fit) was seen between urinary albumin excretion and selectivity index in the diabetic patients, indicating that the capability of differentiating between macromolecules of different charges is again lost with increasing urinary albumin excretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on kidney sialidase in normal and diabetic rats

Rat kidney cortex sialidase was studied using alpha-sialyl-(2----3)-[3H]lactitol and alpha-sialyl-(2----6)-[3H]lactitol as substrates. The enzyme was found mainly in the lysosomal fraction. Only 23% of the sialidase activity of this fraction could be solubilized by a combination of freezing-thawing, sonication and Triton X-100 treatment. The optimal pH for the lysosomal enzyme activity was 4.2 and the enzyme's Km values for alpha-sialyl-(2----3)-lactitol and alpha-sialyl-(2----6)-lactitol were 0.28 and 0.41 mM, respectively. The specific activity was twice as high with the former substrate than with the latter. Sialidase activities in dialyzed kidney cortex homogenates of streptozotocin-diabetic rats and of age-matched control rats were compared. The specific activity was found to be significantly increased in the diabetic animals when using both substrates 5950 +/- 720 (S.E.) dpm/h per mg protein (n = 7) vs. 3970 +/- 370 in the controls (n = 8) with alpha-sialyl-(2----3)-lactitol (P less than 0.025) and 2870 +/- 300 vs. 1820 +/- 170 with alpha-sialyl-(2----6)-lactitol (P less than 0.02). The activities were also found to be increased when expressed per whole kidney cortex (P less than 0.005 and P less than 0.001, respectively). The elevated sialidase activity in diabetic kidney cortex may be related to the reported decrease in sialic acid content of the glomerular basement membrane, which lowers its negative charges and which may contribute to an increased permeability to proteins.

Nonenzymatic glycosylation of basement membrane collagen in diabetes mellitus

For a better understanding of the processes leading to diabetic microangiopathy, type IV collagen from kidneys of patients with long-term diabetes was compared with the collagen from kidneys of sex- and age-matched controls. Type IV collagen from diabetic kidneys revealed no abnormalities in amino acid composition, hydroxylation of proline and lysine, enzymatic glycosylation of hydroxylysine, and immunological reactivity with several monoclonal and polyclonal, anti-type IV collagen antibodies. However, ketoamine-linked hexose, resulting from the nonenzymatic condensation of glucose with lysyl or hydroxylysyl residues, was 1.7-fold higher in diabetic type IV collagen. The stoichiometry of this modification was estimated to be 1-2 residues of hexose per triple helical molecule (Mr 380,000). This small amount of ketoamine-linked hexose might hardly have an effect on the function and turnover of type IV collagen, unless it is bound to a crucial site along the collagen molecule. The nonenzymatic glycosylation of collagen might therefore be a mere consequence of the metabolic disturbances, rather than the primary cause for the late complications of diabetes mellitus.